My children had to get vaccines for hepatitis A and B. Why isn't there a vaccine for hepatitis C?
Efforts to develop a hepatitis C vaccine started more than 20 years ago, when the hepatitis C virus was identified. Since then, researchers have studied several potential vaccines in animals.
Only one vaccine is currently being tested in people. Completion of that vaccine trial is expected in 2016. If it shows that the vaccine prevents chronic hepatitis C, larger trials will still be necessary to verify this effect and determine the best way to deploy the vaccine.
Progress has been slow for a number of reasons, including:
Unique characteristics of the virus
The hepatitis C virus is more variable than are the viruses that cause hepatitis A and hepatitis B. For starters, hepatitis C virus occurs in at least six genetically distinct forms (genotypes), with multiple subtypes. About 50 subtypes have been identified.
The most common hepatitis C virus genotypes vary by geography. Hepatitis C type 1 virus is most common in the United States and Europe. Type 3 is most common in India, the Far East and Australia. And type 4 is most common in Africa and the Middle East. A global vaccine would have to protect against all these variants of the virus.
Limited animal models of hepatitis C infection
Hepatitis C virus can infect chimpanzees, and the infection in chimpanzees is similar to that in adults. Ethical and cost concerns, however, limit medical research with these animals. In the past decade, genetically modified mice with grafted human liver cells have proved to be valuable models for study of hepatitis C infection.
Difficulty enrolling at-risk people in vaccine trials
The only way to find out if a vaccine prevents an infection is to test it in people who are at risk. In developed countries, those at highest risk of hepatitis C infection are people who use needles to inject street drugs. Because injection drug users tend to avoid health care and be difficult to keep in touch with, it can be expensive and time-consuming to enroll, treat and follow them.
A much larger population is at risk of hepatitis C in developing countries, where infection continues to spread via blood transfusions and reuse of supplies in hospitals. The same thing that causes rising rates of infection in developing countries — lack of health care facilities and staff — also makes vaccine trials extremely difficult.
Lack of a clear marker for protection
After you've had hepatitis A or hepatitis B, your immune system "remembers" the virus and keeps you from getting infected again. The hepatitis A and B vaccines work by provoking this protective memory of the viruses without causing illness.
Hepatitis C infection is different. It rarely causes serious problems until you've been infected — often unknowingly — for many years. During those years, the virus escapes the body's immune defenses, which normally limit the course of infection and protect against reinfection. This means the types of vaccines developed for hepatitis A and hepatitis B viruses won't work against hepatitis C virus.
Clues to an approach that might work have come from studies of the 30 percent of people whose immune systems get rid of the hepatitis C virus during the acute stage of infection — roughly the first six months after exposure to the virus. So far, however, researchers haven't found out exactly what immune system product or variation protects these people. Nor do they know exactly how this protective factor stops the virus before it becomes chronic. Answers to these questions may emerge in the next few years, fueling faster progress in hepatitis C vaccine development.